Another Newbie Queastion

Jerry Fike

I've been trying to figure this out for a while. I have some code that looks like this:

PictureBox p = null;
PictureBox pb_dice = null;

for(int i = 1l I < 7; i++)
{
    p = this.Controls["p" + i.ToString()] as PictureBox;
    pb_dice = this.Controls["p_dice" + i.ToString()] as PictureBox;
    p.Image = WindowsFormsApplication1.Properties.Resources.diceBlank;
    pb_dice.Image = WindowsFormsApplication1.Properties.Resources.diceBlank;
}

When I run it, I get an error "Object reference not set to an instance of an object." for the pb_dice.Image, but the p.Image works fine. The resource diceBlank.png is valid and I double-
checked that both Pictureboxes are indeed pictureboxes. Both pictureboxes properties are identical and neither are part of a group or panel.

I'm using Visual Studio C# 2010 Express.
Any suggestions?

7 Replies

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Sam Hobbs16y ago
Accepted answer
Yes we often overlook easy things expecting the problem to be more complicated. It happens.

So at least my suggestion to check for null was relevant; also checking to ensure that there really is a control with the expected id. I know that problems such as that happen.
Sam Hobbs16y ago
Yes the debugger can really help at times. I see many questions about problems that are not described adequately and the debugger could show some useful information. Since you have not had experience with the debugger, it is understandable that you were not familiar with it's use in a situation such as this. I am sure you will use the debugger many more times in the future.
Jerry Fike16y ago
Yes, that's so true.

What also helped me was a tool I hadn't noticed before. In the debug watch window I noticed a little "Text Visualizer" icon in the view column. I clicked on it and saw "p_dice1" (instead of "pb_dice") and then it hit me.
Jerry Fike16y ago
Sam, I'm an idiot!

I looked and looked at that code and didn't see the obvious.

        pb_dice = this.Controls["p_dice" + i.ToString()] as PictureBox;
                                               |
                                               |
                                            should be "pb_dice"

That's what I get for "copying and pasting" to save time. I had a bunch of Pictureboxes named p1, p2
p3, etc. I would usually use pb_1, pb_2, etc, but there were so many it was easier using the shorter name. Then when I created pb_dice1, 2, etc., I copied the p1, p2 ... code with the intent of modifying the names. Of course I missed the "pb" part!

Thanks for your help and sorry I took up your time.
Sam Hobbs16y ago
So let's not dwell on the inaccuracy of the code that was posted. Let's dwell on the error. What is the error message and what line of code is it on?
Jerry Fike16y ago
Not sure why using for(int i = 1; i < 7; i++) is worse than for(int i = 0; i < 6; i++)? The names for
the picturebox dice are pb_dice1 thru pb_dice6. Sure, I could reference them as "pb_dice " + i+1ToString(), but it seems it's a little easier to understand the way I did it (and besides, I did the same thing for p.Image.)

[I see what you mean -- I made a mistake when I posted the first message. The code is really
for(int i = 1; i < 7; i++), not for(int i = 1l I < 7; i++). Not sure how that "|" got in there, and actually the p.Image wouldn't work under that code. I made the mistake typing in the message, not in the actual code.]

It errors on the first iteration of the for loop (i = 1).
Sam Hobbs16y ago
Add an if statement to check pb_dice is null before using it. If it is null then issue an error message and get out, or something; do that as a permanent improvement for reliability purposes. For immediate diagnostic puposes, look for the third or the tenth time or whatever that there is a control with the id "p_dice1".

Also, are you totally sure you posted the correct code? If so, then are you sure you looked closely at it? The code you posted is wrong; you don't want to set i to "1l I < 7".